In a conventional well several casing strings are run into the wellbore as drilling proceeds, whereby each subsequent casing must pass through the previous casing, and therefore must be of smaller diameter than the previous casing. A consequence of such scheme is that the available wellbore diameter through which tools or fluids can pass, becomes stepwise smaller.
It has been proposed to alleviate this problem by installing each subsequent casing in a manner that a relatively short upper end portion thereof extends into the previous casing, and thereafter radially expanding the subsequent casing to an inner diameter substantially equal to the inner diameter of the previous casing. Since the upper end portion of the subsequent casing extends into the lower end portion of the previous casing, the two overlapping portions must be expanded simultaneously. Consequently the expansion force/pressure required to expand these overlapping portions is significantly higher than for the remainder of the lower casing, therefore there is an increased risk that the expander becomes stuck in the overlapping portions of the casings. Also, in case of hydraulic expansion, there is a risk that the fluid pressure required to move the expander through the overlapping portions rises to an unacceptably high level causing the already expanded casing section to fail (e.g. connector failure or pipe burst).